Immersion measuring probes of the general type involved herein are disclosed in a number of prior art patents. For example, see Belgian patent Nos. 828,572; 881,886; 884,405; and 889,276. The prior art patents are generally characterized by a measuring head supported at one end of a paperboard tube. The paperboard tube is provided for its insulating protection when the probe is immersed into molten metal.
An oxygen probe supported by a quartz sheath of uniform wall thickness is taught by British Patent No. 1,094,180. A protective sheath of quartz is objectionable since quartz is transparent to thermal radiation. An oxygen probe supported by a quartz tube partially protected by a tapered graphite sleeve is taught by German Patent No. 1,928,845. The use of graphite as a protective sleeve is objectionable since it combines with oxygen with the result that the graphite sleeve burns off.
It has been found that the prior art probes of the type involved herein are inaccurate due to several features relating to the manner in which said prior art probes are constructed. A large number of solutions involving changes of material as well as changes in construction were investigated. In order to make a satisfactory probe which will give uniform accurate results, it was ascertained that the probe must meet the following criteria:
(a) A considerable reduction of the mass in the vicinity of the measuring was needed to diminish the cooling effect on the metal and thus enable more accurate measurements to be made quicker and at lower temperatures;
(b) When the probe includes a thermocouple, the temperature difference between its cold joints during immersion and temperature meaurement should be reduced;
(c) When the probe includes an oxygen sensor, it should be a solid electro-chemical cell and means should be provided to minimize the influence of oxygen liberated from the oxygen sensor so as to prevent liberated oxygen and other gases from being trapped adjacent to the sensor and thereby giving erroneous readings.
The probe of the present invention is directed to a solution of said problems.